Hi all, truck is a '99 5.9l 4x4 with 115,000 miles.
My truck is burning a lot of oil (about a half quart every week/300 miles). I am running castrol 5w30 full synthetic oil. I switched to synthetic a couple months ago and that's when I started to notice that the truck was burning oil. Could it be that the synthetic oil is causing this? Or maybe too thin of an oil (though, it has been very cold here the last few months)? I was wondering if this could be from a blown plenum. In addition to burning oil, there's a lot of spark knock, and the truck feels a little sluggish in general. I'm gonna do another vacuum test tonight but I'm not sure if it's the plenum since I don't see any oil in the intake. If it matters, I only see blue smoke coming out of the exhaust when I first start the truck up, I have never noticed any when its idling normally or when I'm accelerating. Any info is appreciated, thanks in advance.

 

What you describe about using oil, spark knock and loss of power, is exactly what a blown plenum gasket does. Check it this way, http://dodgeram.info/tsb/2000/09-05-00.htm . You may have that and another prob since it smokes when you start it up, that usually means the valve stem seals are shot. BTW, if the plenum is blown, have the cat checked .

 

Smoke on start up can also be worn/bad rings.

 

+1 zman
Sounds like that lighter, synthetic motor oil is making its way past the valve stem seals and guides. Plenum gasket is probably blown as well.

 

where are the guides located, im planning on doing the valve stem seals and want to replace those too ( could i replace them with the heads on)

 

if a high milage engine has lived its entire life on conventional oil, its a good idea to stay with it. over time, oil slidge will buildup in little crackes and crevasses inside your engine usually around gaskets and seals, Synthetics tend to have a higher detergent content, and when you switch, the new oil will start to wash all that sludge away, uncovering bad gaskets and developing oil leaks.

I would also use a thicker oil with an older engine, i like 15-40, keeps everything nice and tight.

 

the guides are something that a machine shop will have to do with the heads off.....i believe u can replace the seals with the heads on

 

you can do seals with the heads on, but you need shop air and the tool to put into the spark plug threads

 

Your opening up a can of worms by switching to synthetic this late in the game. I did the same as you at 120,000 miles and now my front and rear seals are leaking as well as my valve covers. My engine use to be bone dry prior to synthetic.

 

ive got 115k and switched to syn with no problems